The present invention relates generally to a system and method for controlling a fuel vapor recovery system in an internal combustion engine.
Motor vehicles of recent years typically incorporate a fuel vapor recovery system in connection with the internal combustion engine to reduce the amount of fuel vapors released into the atmosphere from the fuel tank. Typically, a canister containing a fuel vapor absorbing material, such as activated charcoal, is coupled between the fuel tank and the air/fuel intake of the engine. The fuel vapor absorbing material absorbs fuel vapor from the fuel tank. A purge valve positioned between the canister and air/fuel intake of the engine enables the periodic purging of fuel vapors from the canister. The purged fuel vapors are channeled into the air/fuel intake of the engine.
To minimize the emission of hydrocarbons into the atmosphere, it is known to include devices, such as three-way catalysts and hydrocarbon traps, in the exhaust system of the vehicle. These emission control devices generally require that the engine air/fuel ratio be maintained within a certain range to function optimally. Further, favorable vehicle drivability characteristics require that the engine air/fuel ratio be maintained within certain limitations. Accordingly, to maintain favorable drivability characteristics (i.e., avoid transient torque fluctuations) and to limit undesirable emissions, an engine controller typically determines a desired engine air/fuel ratio based on various parameters. The controller generally adjusts the amount of fuel supplied to the engine to maintain the desired air/fuel ratio.
The process of purging the vapor recovery canister causes fuel vapor to be delivered to the engine air/fuel intake, thus causing the air/fuel ratio delivered to the engine cylinders to be altered. Therefore, to avoid increased hydrocarbon emissions and to maintain favorable vehicle drivability characteristics, the addition of fuel vapors into the engine air/fuel intake from the vapor recovery system should be controlled, and the amount of fuel provided by the fuel injectors should be adjusted to account for the addition of fuel vapors from the canister.
Known systems and methods for controlling the amount of fuel provided by the vapor recovery system depend upon the system xe2x80x9clearningxe2x80x9d the amount of fuel flow through the vapor recovery system purge valve each time the position of the purge valve is changed. In particular, the purge valve starts out closed, and the engine controller causes it to be opened gradually. As the purge valve is opened, the engine controller uses an adaptive algorithm to evaluate various feedback parameters and xe2x80x9clearnxe2x80x9d the amount of fuel being provided by the vapor recovery system at each valve position. The controller adjusts the desired amount of fuel provided to the engine from the fuel injectors based thereon. Because the steps of the adaptive algorithm are reactivexe2x80x94the effects of opening the purge valve a given amount must be observed and evaluated each time the valve position is changedxe2x80x94this method is relatively slow to open the purge valve. To maintain adequate storage capability of the canister, it is desirable to be able to purge the canister relatively quickly. Accordingly, the inventors hereof have recognized that a new system and method for controlling vapor recovery system purging that is able to predict the amount of fuel vapor flow based on the purge valve position, as opposed to adaptively xe2x80x9clearningxe2x80x9d the amount of fuel vapor flow, would be enable the purge valve to be opened more quickly, making the system more responsive and robust.
The invention relates to a new system and method for controlling the amount of fuel vapors delivered to the engine cylinders from the fuel vapor recovery system. First, a purge fuel fraction is calculated. The purge fuel fraction is indicative of the relative portion of fuel vapors to the total flow, including air, from the vapor recovery system. The inventors have recognized that the purge fuel fraction remains fairly constant with purge valve position as compared to the purge fuel flow rate, which various prior art methods for controlling purge flow rely upon. The purge fuel fraction calculated in the present invention is used to determine a desired purge fuel amount to be delivered from the vapor recovery system. In this way, the invented system predictively estimates the amount of fuel vapor that the vapor recovery system delivers to the engine instead of reactively xe2x80x9clearningxe2x80x9d such amount after the purge valve is adjusted. As a result, the present invention facilitates opening the purge valve more quickly than prior art systems, and it enables the system to adjust the amount of fuel vapor flow more responsively. Consequently, the system is better able to predict the amount of fuel that is provided by the vapor recovery system and more quickly releases the vapor from the canister to ensure continuing storage capacity.